Fundamental Investigation of the Factors Controlling the CO2 Permeability of Facilitated Transport Membranes Containing Amine-Functionalized Task-Specific Ionic Liquids

Task-specific ionic liquid (IL)-based facilitated transport membranes were prepared with tetrabutylphosphonium amine-functionalized glycinate or 2-cyanopyrrolide ILs. CO2 permeabilities and viscosity and CO2 absorbance of ILs were evaluated. Viscosities of glycinate- or methylglycinate-containing ILs drastically increased with CO2 absorption, probably from hydrogen bonding between their CO2 complexes. The temperature dependence of the CO2 permeability of the IL-based membranes was opposite to that of the viscosity of the corresponding Its. The CO2 permeability at low temperature under dry conditions was improved by using tetrabutylphosphonium 2-cyanopyrrolide, which barely forms hydrogen bonds among its CO2 complexes. However, above 363 K, the tetrabutylphosphonium 2-cyanopyrrolide-based membrane had a lower CO2 permeability than the tetrabutylphosphonium glycinate-based membrane because tetrabutylphosphonium 2-cyanopprolide absorbed less CO2. The major factors controlling CO2 permeability of task-specific IL-based facilitated transport membranes appear to be the viscosity of the CO2 complex (which is the most important factor) and the amount of CO2 absorbed.